Holdfast Training Services Trial Blog

Temporary works are becoming somewhat of a theme on my site, we are moving in to the construction phase with piles and other groundworks appearing all over. I was asked to assist one of the section engineers with a design clash issue on the retaining wall I have written about previously, here is a picture to jog your memory.

The blue wall has been temporarily propped with two levels of 45 degree square hollow sections that also prop off the existing floor slab. The design was intended to contain the lower prop and the upper prop would be removed, as shown in the Temporary propping arrangement.

Somewhere along the line, a mistake has been made either in the fabrication and installation of the props, or in the design of the wall. The lower prop now protrudes at the corner and is also required to be removed. With the base being poured in two levels (a mass concrete pour and a waterproof topper) these can be removed after the first pour.

Essentially the first base pour will replace the action of the lower prop so these can be removed. Our TW designer has asked the base concrete achieve 2/3 of the design compressive strength (approx 26.5N/mm2) before these are removed. Although in a gravity retaining wall, such as this, what affect does the strength of the concrete have?

I have a theory that the TW designer has tried to mitigate against the concrete being stripped out if it doesn’t reach the required compressive strength after the lower props have been removed. This would leave us up the creek without a paddle, as the upper props could not sustain the loading from the existing wall.

Update:

The props have now been removed and the wall hasn’t collapsed.